Optical scanning device

ABSTRACT

A optical scanning device ( 1 ) for scanning an information layer ( 2 ) with a radiation beam ( 25 ) in a writing mode and a reading mode comprises a radiation source ( 7 ) for emitting the beam and an objective lens ( 10 ) for converging the beam so as to form a scanning spot ( 19 ) in the information layer. The device also includes a scanning spot power switch ( 20 ) for switching the size of the cross-section of the beam between a first size at the writing mode and a second, larger size at the reading mode so as to switch the rim intensity of the beam between a first intensity level (Irim,writing) at the writing mode and a second, higher intensity level (Irim,reading) at the reading mode, thereby switching the light power of the scanning spot between a first power level (Pwriting) at the writing mode and a second, lower power level (Preading) at the reading mode.

This application is a 371 of PCT/IB03/03935 filed Sep. 12, 2003

The present invention relates to an optical scanning device for scanningan information layer by means of a radiation beam in a writing mode anda reading mode, the device comprising:

a radiation source for emitting said radiation beam,

an objective lens having an optical axis, for converging said radiationbeam so as to form a scanning spot in the position of said informationlayer, and

a scanning spot power switch arranged in the optical path of saidradiation beam, for switching the light power of said scanning spotbetween a first light power at the writing mode and a second, lowerlight power at the reading mode.

The present invention also relates to a scanning spot power switchsuitable for an optical scanning device for scanning an optical recordcarrier by means of a radiation beam at a writing mode and a readingmode.

“Scanning an information layer” refers to scanning by means of aradiation beam for reading information in the information layer(“reading mode”) and/or writing information in the information layer(“writing mode”). By extension, a writing mode may consist in erasinginformation in the information layer (“erase mode”).

An optical scanning device for scanning an information layer by means ofa radiation beam in a writing mode and a reading mode is known from e.g.the U.S. Pat. No. 4,363,116. The known device has an optical axis andcomprises: a radiation source for emitting the radiation beam, anobjective lens system for converging the radiation beam so as to form ascanning spot in the information layer. The beam emitted by theradiation source has a substantially circular cross-section in a planeperpendicular to the optical axis. It is noted that the beam enteringthe objective lens system has a circular cross-section with the samesize (diameter) in both the writing and reading modes.

The known scanning device further includes a scanning spot power switcharranged in the optical path of said radiation beam, for modifying thelight power of said scanning spot so that the spot has a high lightpower at the writing mode and a low light power at the reading mode.Briefly, the spot light power must be high at the writing mode in orderto produce an optically detectable change in the information layer,thereby writing information in the layer, and low at the reading mode inorder not to alter the information written in the layer. The known spotpower switch includes an electro-optical modulator for varying thepolarization of the beam entering the objective lens system and ananalyzer for converting the polarization variation into an intensityvariation of the beam. Therefore, the switching of the spot light poweris achieved by changing the maximum of the intensity of the radiationbeam entering the objective lens system and the maximal intensity of thebeam has a high level at the writing mode and a low level at the readingmode. It is noted that such an increase of the intensity maximum resultsin an unwarranted in the light power consumption of the scanning device.

While the known scanning device provides the radiation beam with thenecessary high light power at the writing mode, the device does notprovide at the reading mode the beam with the necessary high rimintensity.

In the present description a “rim ray” refers to a ray of a radiationbeam entering the objective lens at the rim or border of the entrancepupil of that lens. Also, a “rim intensity” refers to a normalized valueequals to the intensity of the radiation beam entering the objectivelens at the rim or border of the entrance pupil of the objective lens,divided by the maximum of the intensity, i.e. the intensity at thecenter of the beam. In the following and by way of illustration only“high rim intensity” refers to a rim intensity equal to or higher than70% and “low rim intensity” refers to a rim intensity lower than 70%. Itis noted that such a rim ray and intensity is defined when the entrancepupil of the objective lens is fully filled, i.e. when the size of theradiation beam entering the objective lens is larger than the radius ofthe circular entrance pupil of the objective lens.

In respect of the known device it is noted that the rim intensitybecomes a critical parameter at the reading mode. In that mode thedevice is to provide the beam so as to form the scanning spot with asize sufficiently small, i.e. a size that prevents or minimizes thetangential and radial cross-talk when reading the information layer.This may be achieved inter alia when the radiation beam entering theobjective lens has a high rim intensity. By contrast, the rim intensityis of less importance at the writing mode but the total light power ofthe scanning spot becomes a critical parameter. This is due to the factthat during writing mainly the central part of the scanning spot is usedfor producing the highest temperature capable of producing a detectablechange in the information layer. Consequently, in order to obtain such atemperature on the information layer, the light power of the radiationbeam entering the entrance pupil of the objective lens is to be high. Ingeneral, as a result, the rim intensity of the radiation beam enteringthe objective lens is then rather low.

Accordingly, it is an object to provide an optical scanning device forscanning an optical record carrier by means of a radiation beam having ahigh light power level at the writing mode and a high rim intensitylevel at the reading mode.

This object is reached by an optical scanning device of the typedescribed in the opening paragraph wherein, according to the invention,said scanning spot power switch is further arranged for switching thesize of the cross-section of said radiation beam between a first size atthe writing mode and a second, larger size at the reading mode so as toswitch the rim intensity of said radiation beam between a first rimintensity level at the writing mode and a second, higher rim intensitylevel at the reading mode, thereby switching the light power of saidscanning spot between said first light power level at the writing modeand said second light power level at the reading mode.

Thus, the scanning spot power according to the invention switch acts asa beam size (diameter) modifier so that the beam entering the objectivelens has, at the writing mode, the high light power level and the lowrim intensity level and, at the reading mode, the low light power leveland the high rim intensity level, as explained in detail further.

It is noted that beam size modifiers are known in the field of opticalstorage but for other applications.

First, it is known, e.g. from the Japanese patent application JP11-259895, to use a beam size modifier for focussing a radiation beam inthe different information layers of a multilayer disc. Likewise, it isknown, e.g., from the U.S. Pat. No. 4,307,929 a beam modifier includingan liquid crystal lens that is electrically controllable for changingthe refractive index and therefore the light power of the lens. As aresult, the focal length of the liquid crystal lens is variable forfocussing at different information depths, e.g. for scanning a CD-formatdisc and a DVD-format disc. It is noted that in both cases the knownmodifier changes the vergence of the beam entering the objective lens.

Second, it is known, e.g. from the Japanese patent application JP10-269585, to use a beam size modifier for scanning optical recordcarriers having different information densities. “Information density”refers to the amount of stored information per unit area of theinformation layer. That known modifier is arranged for changing thenumerical aperture of the beam entering the objective lens and the sizeof the scanning spot accordingly, depending on the information densityof the disc to be scanned. Other techniques are known for scanningoptical record carriers having different information densities, e.g.super resolution techniques as described e.g. in the Japanese patentapplication JP 2001-307365. It is noted that, in case of a known beamsize modifier using a super resolution technique, the light power of theradiation beam entering the known modifier equals the light power of theradiation beam emerging from the known modifier.

Third, it is known, e.g. from the U.S. Pat. No. 4,734,906, to use a beamsize modifier for reshaping an astigmatic, non-circular beam emitted bya diode laser into a circular beam free of astigmatism. However, thatknown device reshapes the beam in a manner that is optimized for boththe writing and reading modes, without further considerations of therequirements imposed on the scanning spot at the writing and readingmodes. In other words, that known device provides a compromise forreasonably writing and reading the information layer, but not an optimalsolution for both writing and reading modes. Furthermore, that knownbeam size modifier includes an arrangement with two or three opticalelements, which is difficult to move.

Thus, none of the known beam size modifiers are used for shaping thebeam entering the objective lens so that that beam has, at the writingmode, the high light power level and the low rim intensity level and, atthe reading mode, the low light power level and the high rim intensitylevel.

According to another aspect of the invention, the optical scanningdevice further includes a collimator lens arranged between saidradiation source and said scanning spot power switch, and said scanningspot power switch forms a telescope-like arrangement having a switchablefocal length so that the cross-section of said radiation beam has saidfirst size at the writing mode and said second size at the reading mode.

It is noted that a beam modifier including a telescope-like arrangementis known e.g. from the already cited U.S. Pat. No. 4,734,906. However,with the aid of that known telescope-like arrangement, the beam isoptimized for both the writing and reading modes, i.e. the cross-sectionof that beam has the same size for both modes. In other words, thatknown beam modifier is not switchable between the writing and readingmodes.

It is another object of the present invention to provide a scanning spotpower switch suitable for an optical scanning device for scanning anoptical record carrier by means of a radiation beam having a high lightpower level at the writing mode and a high rim intensity level at thewriting mode.

This object is reached by a scanning spot power switch suitable for anoptical scanning device for scanning an optical record carrier by meansof a radiation beam at a writing mode and a reading mode, the powerswitch being arranged for switching the light power of said radiationbeam between a first light power level at the writing mode and a second,lower light power level at the reading mode wherein, according to theinvention, the power switch is further arranged for switching the sizeof the cross-section of said radiation beam between a first size at thewriting mode and a second, larger size at the reading mode in order toswitch the rim intensity of said radiation beam between a first rimintensity level at the writing mode and a second, higher rim intensitylevel at the reading mode, thereby switching the light power of saidscanning spot between said first light power level at said first modeand said second light power level at said second mode.

The objects, advantages and features of the invention will be apparentfrom the following, more detailed description of the invention, asillustrated in the accompanying drawings, in which:

FIG. 1 is a schematic illustration of components of the optical scanningdevice according to the invention,

FIGS. 2A and 2B show two respective curves representing the intensity atthe entrance pupil of the power switch of the scanning device shown inFIG. 1 operating at either the writing or reading mode,

FIGS. 3A and 3B show two respective curves representing at the writingmode the intensity at the exit pupil of the power switch of the scanningdevice shown in FIG. 1 operating at the writing mode,

FIGS. 4A and 4B show two respective curves representing the intensity atthe exit pupil of the power switch of the scanning device shown in FIG.1 operating at the reading mode,

FIG. 5 shows a cross-section of a first embodiment of the power switchshown in FIG. 1,

FIG. 6A shows certain optical components of the scanning device shown inFIG. 1 provided with the power switch shown in FIG. 5, operating at thewriting mode,

FIG. 6B shows certain optical components of the scanning device shown inFIG. 1 provided with the power switch shown in FIG. 5, operating at thereading mode,

FIG. 7 shows a schematic cross-section of the power switch shown in FIG.5,

FIG. 8 shows a cross-section of a second embodiment of the power switchshown in FIG. 1,

FIG. 9 shows a third embodiment of the power switch shown in FIG. 1,

FIG. 10A shows certain optical components of the scanning device shownin FIG. 1 provided with the power switch shown in FIG. 9, operating atthe writing mode, and

FIG. 10B shows certain optical components of the scanning device shownin FIG. 1 provided with the power switch shown in FIG. 9, operating atthe reading mode.

FIG. 1 is a schematic illustration of components of the optical scanningdevice according to the invention, designated by the numeral reference1. The optical scanning device 1 is capable of scanning at least oneinformation layer 2 of at least one optical record carrier 3 by means ofa radiation beam 4 in a first, writing mode and in a second, readingmode.

By way of illustration, the optical record carrier 3 includes atransparent layer 5 on one side of which the information layer 2 isarranged. The side of the information layer facing away from thetransparent layer 5 is protected from environmental influences by aprotective layer 6. The transparent layer 5 acts as a substrate for theoptical record carrier 3 by providing mechanical support for theinformation layer 2. Alternatively, the transparent layer 5 may have thesole function of protecting the information layer 2, while themechanical support is provided by a layer on the other side of theinformation layer 2, for instance by the protective layer 6 or by anadditional information layer and transparent layer connected to theuppermost information layer. It is noted that the information layer hasan information layer depth 7 that corresponds to the thickness of thetransparent layer 5. The information layer 2 is a surface of the carrier3. That surface contains at least one track, i.e. a path to be followedby the spot of a focused radiation on which path optically-readablemarks are arranged to represent information. The marks may be, e.g., inthe form of pits or areas with a reflection coefficient or a directionof magnetization different from the surroundings.

The optical scanning device 1 includes a radiation source 7, acollimator lens 8, a first beam splitter 9, a scanning spot power switch20, an objective lens 10 having an optical axis 11, and a detectionsystem 12. Furthermore, the optical scanning device 1 includes aservocircuit 13, a focus actuator 14, a radial actuator 15, and aninformation processing unit 16 for error correction.

In the following “Z-axis” corresponds to the optical axis 11 of theobjective lens 8. “O” is the point of intersection between the opticalaxis 11 and the information plane 2. In the case where the opticalrecord carrier 3 has the shape of a disc, the following is defined withrespect to a given track: the “radial direction” is the direction of areference axis, the X-axis, between the track and the center of the discand the “tangential direction” is the direction of another axis, theY-axis, that is tangential to the track and perpendicular to the X-axis.It is noted that (O, X, Y, Z) forms an orthogonal base associated withthe position of the information plane 2.

The radiation source 7 supplies the radiation beam 4 at a wavelength λ.For example, the radiation source 7 comprises a semiconductor laser forsupplying the radiation beam 4.

The collimator lens 8 is arranged along the optical path of theradiation beam 4 and, in that embodiment, between the radiation source 7and the beam splitter 9. The collimator lens 8 transforms the radiationbeam 4 into a substantially collimated beam 17. The collimator lens 8has an optical axis that is the same as the optical axis 1 of theobjective lens 10.

The first beam splitter 9 is arranged between the radiation source 7 andthe power switch 20 and, in that embodiment, the collimator lens 8 andthe power switch 20. The splitter 9 transmits the collimated radiationbeam 17 toward the objective lens 10. Preferably, the beam splitter 9 isformed with a plane parallel plate that is tilted with an angle α withrespect to the Z-axis and, more preferably, α=45°.

The scanning spot power switch 20 is arranged in the optical path of theradiation beam 17 and, in the embodiment shown in FIG. 1, between thecollimator lens 8 and the objective lens 10. The power switch 20 has anentrance pupil plane 20 a facing the beam splitter 9 and an exit pupilplane 20 b facing the objective lens 10. In the embodiment shown in FIG.1 “O₁” is the point of intersection between the optical axis 11 and theentrance plane 20 a, “X₁-axis” and “Y₁-axis” are two axes of theentrance plane 20 a that are orthogonal to each other, and “Z₁-axis” isthe axis normal to the entrance plane 20 a and passing through the pointO₁. It is noted that (O₁, X₁, Y₁, Z₁) forms an orthogonal baseassociated with the position of the entrance plane 20 a. It is alsonoted in the embodiment shown in FIG. 1 that the entrance plane 20 a iscentered on the optical axis 11 of the objective lens 10: the X₁-, Y₁-and Z₁-axes are therefore parallel to the X-, Y- and Z-axes,respectively. Likewise, “O₂” is the point of intersection between theoptical axis 11 and the exit plane 20 b, “X₂-axis” and “Y₂-axis” are twoaxes of the exit plane 20 b that are orthogonal to each other, and“Z₂-axis” is the axis normal to the exit plane 20 b and passing throughthe point O₂. It is noted that (O₂, X₂, Y₂, Z₂) forms an orthogonal baseassociated with the position of the exit plane 20 b. It is also noted inthe embodiment shown in FIG. 1 that the exit plane 20 b is centered onthe optical axis 11 of the objective lens 10: the X₂-, Y₂- and Z₂-axesare therefore parallel to the X-, Y- and Z-axes, respectively.

The power switch 20 is arranged for modifying the light power of thescanning spot 19 so that the spot has a first light power levelP_(writing) at the writing mode and a second, lower light power levelP_(reading) at the reading mode. Thus, the power switch 20 transformsthe radiation beam 17 entering the entrance plane 20 a into a radiationbeam 25 emerging from the exit plane 20 b, where the radiation beam 17has an intensity I₁ in the entrance plane 20 a at both writing andreading modes and the radiation beam 25 has, in the exit plane 20 b, anintensity I_(2,writing) at the writing mode and a different intensityI_(2,reading) at the reading mode. The incident beam 17 has the sameintensity profile (I₁) at both writing and reading modes and theemerging beam 25 has two different intensity profiles, I_(2,writing) atthe writing mode and I_(2,reading) at the reading mode. The power switch20 is described further in detail.

The objective lens 10 transforms the radiation beam 25 (that issubstantially collimated in the embodiment shown in FIG. 1) to a focusedradiation beam 18 so as to form a scanning spot 19 in the position ofthe information layer 2. In the embodiment shown in FIG. 1 the lens 10has an entrance pupil 10 a and an exit pupil 10 b that are rotationalsymmetric with respect to the optical axis 11: the entrance pupil 10 ahas a circular rim or border. In the following “r_(o)” is the radius(positive value) of the entrance pupil 10 a and by way of illustrationonly r_(o) is equal to 1.5 mm. It is noted that the objective lens maybe formed as a hybrid lens such as a lens combining refractive elements,used in an infinite-conjugate mode. Such a hybrid lens may be formed bymeans of either a diamond turning process or a lithographic processusing the photopolymerization of, e.g., an UV curing lacquer. It is alsonoted that the objective lens 10 shown in FIG. 1 is formed as aconvex-convex lens; however, other lens element types such asplano-convex or convex-concave lenses can be used. Furthermore, theoptical scanning device 1 may include a pre-objective lens (not shown inFIG. 1 but shown by way of illustration only in FIGS. 6A and 6B anddesignated with the reference numeral 10′). Such a pre-objective lens isarranged between the collimator lens 8 and the objective lens 10 so asto form a compound objective lens system. Alternatively, the objectivelens system may contain more than one pre-objective lens.

During scanning the record carrier 3 rotates on a spindle (not shown inFIG. 1) and the information layer 2 is then scanned through thetransparent layer 5. The focused radiation beam 18 reflects on theinformation layer 2, thereby forming a reflected beam 21 which returnson the optical path of the forward converging beam 18. The objectivelens 10 transforms the reflected radiation beam 21 to a reflectedsubstantially collimated radiation beam 22. The beam splitter 9separates the forward radiation beam 17 from the reflected radiationbeam 22 by transmitting at least a part of the reflected radiation beam22 towards the detection system 12.

The detection system 12 includes a convergent lens 23 and a quadrantdetector 24 for capturing said part of the reflected radiation beam 22.The quadrant detector 24 converts the part of the reflected radiationbeam 22 to one or more electrical signals. One of the signals is aninformation signal I_(data), the value of which represents theinformation scanned on the information layer 2. The information signalI_(data) is processed by the information processing unit 16 for errorcorrection. Other signals from the detection system 12 are a focus errorsignal I_(focus) and a radial tracking error signal I_(radial). Thesignal I_(focus) represents the axial difference in height along theZ-axis between the scanning spot 19 and the position of the informationlayer 2. Preferably, the signal I_(focus) is formed by the “astigmaticmethod” which is known from, inter alia, the book by G. Bouwhuis, J.Braat, A. Huijser et al, entitled “Principles of Optical Disc Systems,”pp. 75–80 (Adam Hilger 1985) (ISBN 0-85274-785-3). The radial trackingerror signal I_(radial) represents the distance in the XY-plane of theinformation layer 2 between the scanning spot 19 and the center of atrack in the information layer 2 to be followed by the scanning spot 19.Preferably, the signal I_(radial) is formed from the “radial push-pullmethod” which is known from, inter alia, the book by G. Bouwhuis, pp.70–73.

The servocircuit 13 is arranged for, in response to the signalsI_(focus) and I_(radial), providing servo control signals I_(control)for controlling the focus actuator 14 and the radial actuator 15,respectively. The focus actuator 14 controls the position of theobjective lens 10 along the Z-axis, thereby controlling the position ofthe scanning spot 19 such that it coincides substantially with the planeof the information layer 2. The radial actuator 14 controls the positionof the objective lens 10 along the X-axis, thereby controlling theradial position of the scanning spot 19 such that it coincidessubstantially with the center line of the track to be followed in theinformation layer 2.

The power switch 20 is now described in further detail. As alreadymentioned, at the writing mode, the power switch 20 transforms theradiation beam 17 having the intensity I₁ in the entrance plane 20 ainto the radiation beam 25 having the intensity I_(2,writing) in theexit plane 20 b so that the light power P of the scanning spot 19 equalsthe high power level P_(writing). At the reading mode, the power switch20 transforms the radiation beam 17 having the same intensity I₁ in theentrance plane 20 a into the radiation beam 25 having the intensityI_(2,reading) in the exit plane 20 b so that the light power P of thescanning spot 19 equals the low power level P_(reading).

FIG. 2A shows a curve 31 representing along the X₁-axis the intensity I₁at the entrance pupil of the power switch 20. FIG. 2B shows a curve 32representing along the Y₁-axis the intensity I₁ at the entrance pupil ofthe power switch 20. As shown in FIGS. 2A and 2B, the intensity I₁ has aGaussian-like profile:

$\begin{matrix}{{I_{1}\left( {x_{1},y_{1}} \right)} = {I_{0}{\mathbb{e}}^{{- {(\frac{x_{1}}{A})}^{2}} - {(\frac{y_{1}}{B})}^{2}}}} & (1)\end{matrix}$where “I₁(x₁,y₁)” is the value of the intensity I₁ at a point ofcoordinates (x₁,y₁) in the Cartesian coordinate system (O₁, X₁, Y₁),“I₀” is the maximum of the intensity I₁ (i.e. the intensity of thecentral ray of the radiation beam 17) and “A” and “B” are two constantparameters that depend inter alia on the radiation source 7. In theembodiment shown in FIG. 1 the parameters A and B also depend on theoptical components arranged between the radiation source 7 and the powerswitch 10, e.g. on the collimator lens 8. In the following and by way ofillustration only the radiation beam 4 emitted from the radiation source7 has an elliptical cross-section and the parameters A and B thereforediffer from each other. For example only, the parameters A and B areequal to 2.68 and 2.24, respectively. It is noted that in the case wherethe radiation beam 17 has a circular cross-section the parameters A andB are equal to each other.

FIG. 3A shows a curve 33 representing along the X₂-axis the intensityI_(2,writing) at the exit pupil 20 b of the power switch 20. FIG. 3Bshows a curve 34 representing along the Y₂-axis the intensityI_(2,writing) at the exit pupil 20 b of the power switch 20. As shown inFIGS. 3A and 3B, the intensity I_(2,writing) has a Gaussian-likeprofile:

$\begin{matrix}{{I_{2,{writing}}\left( {x_{2},y_{2}} \right)} = {I_{0,{writing}}.{\mathbb{e}}^{{- {(\frac{x_{2}}{C})}^{2}} - {(\frac{y_{2}}{D})}^{2}}}} & (2)\end{matrix}$where “I_(2,writing)(x₂,y₂)” is the value of the intensity I_(2,writing)at a point of coordinates (x₂,y₂) in the Cartesian coordinate system(O₂, X₂, Y₂), “I_(0,writing)” is the maximum of the intensityI_(2,writing) (i.e. the intensity of the central ray of the radiationbeam 25 at the writing mode) and “C” and “D” are two constant parametersthat depend on the parameters A and B and on design parameters of thepower switch 20 in respect of the writing mode. In the following and byway of illustration only the parameters C and D are equal to 2.51 and2.10, respectively. It is noted that in the case where the radiationbeam 17 has a circular cross-section the parameters C and D are equal toeach other.

FIG. 4A shows a curve 35 representing along the X₂-axis the intensityI_(2,reading) at the exit pupil 20 b of the power switch 20. FIG. 4Bshows a curve 36 representing along the Y₂-axis the intensityI_(2,reading) at the exit pupil 20 b of the power switch 20. As shown inFIGS. 3A and 3B, the intensity I_(2,reading) has a Gaussian-likeprofile:

$\begin{matrix}{{I_{2,{reading}}\left( {x_{2},y_{2}} \right)} = {I_{0,{reading}}.{\mathbb{e}}^{{- {(\frac{x_{2}}{E})}^{2}} - {(\frac{y_{2}}{F})}^{2}}}} & (3)\end{matrix}$where “I_(2,reading)(x₂,y₂)” is the value of the intensity I_(2,reading)at a point of coordinates (x₂,y₂)in the Cartesian coordinate system (O₂,X₂, Y₂), “I_(0,reading)” is the maximum of the intensity I_(2,reading)(i.e. the intensity of the central ray of the radiation beam 25 at thereading mode) and “E” and “F” are two constant parameters that depend onthe parameters A and B and on design parameters of the power switch 20in respect of the reading mode. In the following and by way ofillustration only the parameters E and F are equal to 2.87 and 2.40,respectively. It is noted that in the case where the radiation beam 17has a circular cross-section the parameters E and F are equal to eachother. It is also noted that the maximum intensities I₀, I_(0,writing)and I_(0,reading) may differ from each other.

More specifically, in order to transform the intensity I₁ into theintensity I_(2,writing) or I_(2,reading), the power switch 20 modifiesthe size of the radiation beam 25 entering the objective lens 10 at thewriting or reading mode. In the present description the “size” of aradiation beam refers, if the beam has an elliptical cross-section, tothe length of the long or short axis and, if the beam has a circularcross-section, to the radius of that circular cross-section. Also in thepresent description the “cross-section” of a radiation beam refers tothe cross-section of the beam in a plane that is perpendicular to thecentral ray of the beam.

Thus, the power switch 20 is arranged so that, at the writing mode, theradiation beam 25 has a first large size in order to have a first lowrim intensity level I_(rim,writing) so that the scanning spot 19 has thehigh power level P_(writing). The power switch 20 is also arranged sothat, at the reading mode, the radiation beam 25 has a second small size(i.e. smaller than said first size) in order to have a second high rimintensity level I_(rim,reading) (i.e. higher than the rim intensitylevel I_(rim,writing)) so that the scanning spot 19 has the low powerlevel P_(reading). In other words, by comparison with the size of theincident beam 17, the power switch 20 decreases the size of the emergingbeam 25 at the writing mode and increases the size of the emerging beam25 at the reading mode.

In the preferred case where the circular entrance pupil 10 a (having theradius r_(o)) of the objective lens 10 is fully filled, the high and lowpower levels P_(writing) and P_(reading) are given by the followingequations:

$\begin{matrix}{P_{writing} = {\int_{{x_{2}},{{y_{2}} \leq r_{o}}}{\int{{I_{2,{writing}}\left( {x_{2},y_{2}} \right)}{\mathbb{d}x_{2}}\ {\mathbb{d}y_{2}}}}}} & \left( {4a} \right) \\{P_{reading} = {\int_{{x_{2}},{{y_{2}} \leq r_{o}}}{\int{{I_{2,{reading}}\left( {x_{2},y_{2}} \right)}{\mathbb{d}x_{2}}\ {\mathbb{d}y_{2}}}}}} & \left( {4b} \right)\end{matrix}$provided that that the light power P of the scanning spot equals thelight power of the radiation beam 25 in the entrance pupil 10 a. Thisoccurs where transmission loss due to absorption of the objective lens10 is negligible.

Furthermore, it is noted that, at the writing mode, any rim ray of theradiation beam 25 comes from a ray of the radiation beam 17 that isentering the entrance plane 20 a at a first point of Cartesiancoordinates (x₁, y₁), wherein the distance between that first point andthe point O₁ equals a first distance h_(writing) that is constantregardless of the ray. Therefore, at the writing mode, the coordinates(x₁, y₁) of that first point are given by the following equation.x ₁ ² +y ₁ ² =h _(writing) ²  (5a)Likewise, at the reading mode, any rim ray of the radiation beam 25comes from a ray of the radiation beam 17 that is entering the entranceplane 20 a at a second point of Cartesian coordinates (x₁, y₁), whereinthe distance between that second point and the point O₁ equals a seconddistance h_(reading) that is constant regardless of the ray. Therefore,at the reading mode, the coordinates (x₁, y₁) of that second point aregiven by the following equation.x ₁ ² +y ₁ ² =h _(reading) ²  (5b)

Thus, it derives from Equations (4a), (4b), (5a) and (5b) that:

$\begin{matrix}{P_{writing} = {\int_{{x_{1}},{{y_{1}} \leq h_{writing}}}{\int{{I_{1}\left( {x_{1},y_{1}} \right)}{\mathbb{d}x_{1}}\ {\mathbb{d}y_{1}}}}}} & \left( {6a} \right) \\{P_{reading} = {\int_{{x_{1}},{{y_{1}} \leq h_{reading}}}{\int{{I_{1,}\left( {x_{1},y_{1}} \right)}{\mathbb{d}x_{1}}\ {\mathbb{d}y_{1}}}}}} & \left( {6b} \right)\end{matrix}$

By properly designing the power switch 20 (as explained in detailfurther) the heights h_(writing) and h_(reading) can be chosen so thatthe rim intensity levels I_(rim,writing) and I_(rim,reading) equaldifferent desired values and therefore the power levels P_(writing) andP_(reading) equal to different desired values. By way of illustrationonly, Table I shows desired values of the rim intensity levelsI_(rim,writing) and I_(rim,reading) along the X₂-axis and the Y₂-axis,the corresponding heights h_(writing) and h_(reading) along the X₁-axisand the Y₁-axis (according to FIGS. 2A and 2B), and the resulting lightpower levels P_(writing) and P_(reading) (according to Equations (6a)and (6b)).

TABLE I I_(rim,writing) I_(rim,reading) h_(writing) h_(reading)P_(writing) P_(reading) X₁- or 70% 75% 1.6 mm 1.4 mm 6.530I₀ 5.242I₀X₂-axis Y₁- or 60% 68% 1.6 mm 1.4 mm Y₂-axis

It is noted in Table I that the ratio P_(writing)/P_(reading)approximately equals 1.25 in absence of transmission loss in the opticalpath of the radiation beam. It is also noted that the rim intensity ofthe radiation beam 25 is lower at the writing mode than at the readingmode and that the power light of the scanning spot 19 is larger at thewriting mode than at the reading mode. Thus, the optical scanning device1 allows scanning of the optical record carrier 3 by means of theradiation beam 25 that has a high light power at the writing mode and ahigh rim intensity at the reading mode.

It is also noted that the power switch 20 forms a telescope-likearrangement having both object and image conjugates at the infinity,where the telescopic arrangement has a switchable transversemagnification between the writing and reading modes. Thus, themagnification telescopic arrangement equals

$\frac{r_{o}}{h_{writing}}$at the writing mode and

$\frac{r_{o}}{h_{reading}}$at the reading mode.

Three embodiments of the scanning spot power switch shown in FIG. 1 arenow described in detail.

The first embodiment 201 of the scanning spot power switch 20, hereafterdesignated by the reference numeral 201, is now described. FIG. 5 showsa cross-section in the Y₁Z₁-plane of the power switch 201. In thatembodiment the power switch 201 comprises two variable focus lenselements 41 and 42 in the form of an electrowetting device 60. It isnoted that the principle of the variable focus lens is described indetail in PH NL020163 and PH NL011095.

The electrowetting device 60 comprises a cylinder 43 of conductivematerial. The cylinder 43 is coated with an insulating layer 44. Theinner side of the cylinder is provided with a fluid contact layer 45.The conductive cylinder 43 forms a common first electrode for the lenselements 41 and 42. The second electrode of the first lens element 41 isconstituted by an annular conductive layer 46 having a centraltransparent area for passing radiation. A conductive layer 47 at theexit side forms the second electrode of the second lens element 42. Twotransparent layers 48 and 49 may cover the conductive layers 46 and 47,respectively. The central portion of the cylinder 43 is filled with afirst, transparent and non-conductive fluid (liquid or vapor) 50. Ateach side of the fluid 50, a second, transparent and conductive, fluid(liquid or vapor) 51 is present. The fluid 51 has a first refractiveindex n₁ and the fluid 50 has a second refractive index n₂. In thatembodiment the first refractive index n₁ is lower than the secondrefractive index n₂. By way of illustration only, in the embodimentshown in FIG. 5, the first fluid 50 is water (n₁=1.349) and the secondfluid 51 is oil, e.g. polydimethyl(8–12%)-phenylmethylsiloxame copolymer(n₂=1.425). Alternatively, the first fluid 50 may be oil and the secondfluid 51 may be water. Also alternatively, the fluid present at the exitside may differ from the fluid present at the entrance side. Thenon-miscible fluids 50 and 51 at the entrance side of the power switch201 (i.e. the side facing the X₁Y₁-plane) are separated by a firstmeniscus 52 which forms the first variable focus lens element 41. Thefluids 50 and 51 at the exit side of the power switch 201 (i.e. the sidefacing the X₂Y₂-plane) are separated by a second meniscus 53 which formsthe second variable focus lens element 42. In the following “R₁” is theradius of curvature of the first meniscus 52 and “R₂” is the radius ofcurvature of the second meniscus 53. The curvature of the menisci andthus the focal distance of the lens elements 41 and 42 can be changedindependently form each other by means of controllable voltage sources54 and 55, respectively. In the following “V₁” is the voltage of thesource 54 and “V₂” is the voltage of the source 55.

In that embodiment decreasing and increasing the size of the radiationbeam 25 at the writing and reading modes, respectively, is performed bychanging the radii of curvature R₁ and R₂ via adaptation of the voltagesV₁ and V₂. FIG. 6A shows certain optical components of the scanningdevice 1 provided with the power switch 201 shown in FIG. 5, operatingat the writing mode. FIG. 6B shows the same optical components operatingat the reading mode.

As shown in FIG. 6A, the fist meniscus 40 has a concave curvature wherethe radius of curvature R₁ is negative and, due to the differencebetween the refractive indices n₂ and n₁, the lens element 41 acts as apositive, converging lens element. The second meniscus 42 has a concavecurvature where the radius of curvature R₂ is negative and, due to thedifference between the refractive indices n₁ and n₂, the second lenselement 42 acts as a negative, diverging lens element.

As shown in FIG. 6B, the fist meniscus 40 has a convex curvature wherethe radius of curvature R₁ is positive and, due to the differencebetween the refractive indices n₂ and n₁, the lens element 41 acts as anegative, diverging lens element. The second meniscus 42 has a convexcurvature where the radius of curvature R₂ is positive and, due to thedifference between the refractive indices n₁ and n₂, the second lenselement 42 acts as a positive, converging lens element.

The distances h_(writing) and h_(reading) depends on the radii ofcurvature R₁ and R₂ and other design parameters of the electrowettingdevice shown in FIG. 5. FIG. 7 shows a schematic cross-section of thescanning spot power switch 201 shown in FIG. 5. The path of a rim ray ofthe radiation beam 25 through the power switch 201 is shown in FIG. 7 bya solid line. The following design parameters are shown in FIG. 7: “d₁”is the thickness of the first fluid 50 along the optical axis 11, “d₂”is the thickness of the second fluid 51 in the entrance side along theoptical axis 11, “d₃” is the thickness of the second fluid 51 in theexit side along the optical axis 11, “d₀” is the thickness of thetransparent layer 48 or 49 along the optical axis 11, and “n₀” is therefractive index of the transparent layer 48 or 49. It has been foundthat, in the paraxial approximation (with a typical accuracy of 20%),the distances h_(writing) and h_(reading) are given, in that embodiment,by the following equations:

$\begin{matrix}{R_{1} = \frac{d_{1}\left( {n_{1} - n_{2}} \right)}{n_{1}\left( {1 - \frac{r_{o}}{h_{1}}} \right)}} & \left( {7a} \right) \\{R_{2} = {R_{1} + {\frac{n_{2} - n_{1}}{n_{1}}d_{1}}}} & \left( {7b} \right)\end{matrix}$where “h₁” is either the distance h_(writing) or the distanceh_(reading).

By way of illustration only, Table II shows values of the radii ofcurvature R₁ and R₂ in the writing and reading modes and thecorresponding heights h_(writing) and h_(reading) that have beenobtained with ray-tracing simulations.

TABLE II h_(writing) R₁ [in mm] R₂ [in mm] [in mm] h_(reading) [in mm]Writing mode −3.7 −3.492 1.6 N/A Reading mode 3.2 3.408 N/A 1.4

Therefore, and with reference to Table I, by a proper choice of theradii of curvature it is possible to transform the radiation beam 17into the radiation beam 25 where the rim intensity of the beam 25entering the objective lens 10 equals, at the writing mode,I_(rim,writing) so that the light power P equals P_(writing) and, at thereading mode, I_(rim,reading) so that the light power P equalsP_(reading). It is noted that, while in the ideal case the ratioP_(writing)/P_(reading) approximately equals 125% (see Table I), thatratio equals 120% in the case where the optical scanning device 1 isprovided with the power switch 201 shown in FIG. 5. This results from atransmission loss of the order of 5% occurring in the second fluid(oil).

Advantageously, the power switch 201 can be made substantially morecompact and consumes substantially less electric power for the switchingaction between the writing and reading modes than a conventional powerswitch. By these properties this embodiment is very suitable to be builtin a miniature device for small and/or handheld and/or battery poweredapparatus, for example a mobile phone, a personal digital assistant(PDA), a personal computer camera, an intercom system and an electronicgame.

It is noted in FIGS. 6A and 6B that the scanning spot power switch 201is arranged between the collimator lens 8 and the objective lens 10,thereby forming a telescope-like arrangement having both object andimage conjugates at the infinity.

The second embodiment of the power switch 20, designated hereafter bythe reference numeral 202, is an alternative of the power switch 201shown in FIG. 5. FIG. 8 shows a cross-section in the Y₁Z₁-plane of thepower switch 202 where the first and second variable focus lens elementsare arranged in two different electrowetting devices 60′ and 60″,respectively.

As shown in FIG. 8, the electrowetting device 60′ comprises a cylinder43′ of conductive material, coated with an insulating layer 44′. Theinner side of the cylinder 43′ is provided with a fluid contact layer45′. The conductive cylinder 43′ forms a common electrode for said firstlens element. The second electrode of said first lens element isconstituted by an annular conductive layer 46′ having a centraltransparent area for passing radiation. The device 60′ is also providedwith two transparent layers 48′ and 49′. The layer 49′ covers theconductive layer 46′. One portion of the cylinder 43′ is filled with thesame fluid than the first fluid 50 of the embodiment shown in FIG. 5.The other portion of cylinder is filled with the same first fluid thanthe second fluid 51 of the embodiment shown in FIG. 5. The non-misciblefluids 50 and 51 are separated by a meniscus 52′ that has the same shapeand radius of curvature than the meniscus 52 shown in FIG. 5, controlledby a voltage source 54′ that provides the same voltage than the voltageV₁ shown in FIG. 5.

Also as shown in FIG. 8, the electrowetting device 60″ comprises acylinder 43″ of conductive material, coated with an insulating layer44″. The inner side of the cylinder 43″ is provided with a fluid contactlayer 45″. The conductive cylinder 43″ forms a common electrode for saidsecond lens element. The second electrode of said second lens element isconstituted by an annular conductive layer 47″ having a centraltransparent area for passing radiation. The device 60″ is also providedwith two transparent layers 48″ and 49″. The layer 49″ covers theconductive layer 47″. One portion of the cylinder 43″ is filled with thesame fluid than the first fluid 50 of the embodiment shown in FIG. 5.The other portion of cylinder is filled with the same first fluid thanthe second fluid 51 of the embodiment shown in FIG. 5. The non-misciblefluids 50 and 51 are separated by a meniscus 53″ that has the same shapeand radius of curvature than the meniscus 53 shown in FIG. 5, controlledby a voltage source 55″ that provides the same voltage than the voltageV₂ shown in FIG. 5.

It is noted that the scanning spot power switch 202 is arranged betweenthe collimator lens 8 and the objective lens 10, thereby forming atelescope-like arrangement having both object and image conjugates atthe infinity.

The third embodiment of the scanning spot power switch 20, hereafterdesignated with the reference numeral 203, is now described. FIG. 9shows the optical components of the power switch 203.

In the embodiment shown in FIG. 9, the radiation beam 4 (and thereforethe radiation beam 25) is linearly polarized along an axis ofpolarization that is parallel to either the X₁-axis or the Y₁-axis. Inthe following “p_(⊥)” is a state of linear polarization along an axisparallel to the X₁-axis and represented by a dot in the figures; “p_(∥)”is a state of linear polarization along an axis perpendicular to theX₁-axis and represented by an arrow in the figures. Also in thatembodiment the first beam splitter 9 is arranged between the radiationsource 7 and the collimator lens 8. Further, the splitter 9 is apolarizing beam splitter so that the radiation beam entering that beamsplitter is transmitted toward the power switch 203 when that beam hasthe polarization p_(∥) and reflected toward the detection system 24 whenthat beam has the polarization p_(⊥).

The power switch 203 comprises a polarization switch 70, a first mirror71, a second mirror 71, a first quarter-wavelength 73, a secondquarter-wavelength 74, a third quarter-wavelength 75 and a secondpolarizing beam splitter 76. The polarizing beam splitter 76 is capableof transmitting and reflecting any radiation beam entering the splitter76 depending on the polarization of that beam: it transmits a beam thathas a polarization p_(∥) and reflects a beam that has a polarizationp_(∥). The first mirror 71 is arranged on one side of the polarizingbeam splitter 76: the optical axis of that mirror is perpendicular tothe optical axis 11, i.e. parallel to the O₁Y₁-axis. The second mirror72 is arranged on another side of the polarizing beam splitter 76: theoptical axis of that mirror is perpendicular to the optical axis 11,i.e. parallel to the O₁Y₁-axis. In that embodiment the mirrors 71 and 72forms a Gaussian-type telescopic arrangement, i.e. the image focal pointof the mirror 71 is the object focal point of the mirror 72. The firstquarter-wavelength plate 73 is arranged between the polarizing beamsplitter 76 and the mirror 71 so as to have the same optical axis as themirror 71. The second quarter-wavelength plate 74 is arranged betweenthe polarizing beam splitter 76 and the mirror 72 so as to have the sameoptical axis as the mirror 72. The third quarter-wavelength plate 75 isarranged between the polarizing beam splitter 76 and the objective lens10 so as to have the same optical axis as the optical axis 11 of theobjective lens 10. The polarization switch 70 is arranged in the opticalpath of the radiation beam transmitted by the first beam splitter 9 and,in that embodiment, between the collimator lens 8 and the second beamsplitter 76. The polarization switch 70 is capable of changing thepolarization of that radiation beam between the polarizations P_(⊥) andp_(∥). The polarization switch 70 comprises, by way of illustrationonly, an electrically controllable liquid crystal cell. In theembodiment shown in FIG. 9 the polarization switch 70 is arranged sothat the polarization of the radiation beam going to and coming from thesecond polarizing beam splitter 76 is switched at the writing mode andis not changed at the reading mode.

In that embodiment decreasing and increasing the size of the radiationbeam 25 at the writing and reading modes, respectively, is performed bychanging the optical path of the radiation beam propagating through thepower switch 203 via switching of the polarization of that beam. FIG.10A shows certain optical components of the scanning device 1 providedwith the scanning spot power switch 203 shown in FIG. 9, operating atthe writing mode. FIG. 10B shows the same optical components operatingat the reading mode. The paths of a rim ray of the radiation beam 25, inparticular through the power switch 203, are shown in FIG. 10A (writingmode) and FIG. 10B (reading mode) by dotted lines from the radiationsource 7 to the record carrier 3 and by solid lines from the recordcarrier 3 to the detector 24.

As shown in FIG. 10A (writing mode), the radiation beam 4 has thepolarization p_(∥) and therefore the polarizing beam splitter 9transmits that beam toward the collimator lens 8. The collimated beam 17with the polarization p_(∥) enters the power switch 203 and is enteringthe polarization switch 70. In that embodiment and at the writing modethe polarization switch 70 changes the polarization p_(∥) to thepolarization p_(⊥). Thus, the radiation beam entering the secondpolarizing beam splitter 76 has the polarization p_(⊥): the splitter 76reflects that beam toward the mirror 71 via the plate 73. The mirror 71then reflects the beam toward the beam splitter 76, again via the plate73, so that the beam has now the polarization p_(∥). Consequently, thebeam splitter 76 transmits that beam toward the second mirror 72 via theplate 74. The mirror 72 then reflects the beam toward the beam splitter76, again via the plate 74, so that the beam has now the polarizationp_(⊥). Thus, the beam splitter reflects that beam toward the objectivelens 10 via the plate 75. After reflection on the information layer 2,the beam propagates to the beam splitter 76 via the objective lens 10and the plate 75, so that the beam has now the polarization p_(∥).Therefore, the beam splitter 76 transmits toward the polarization switch70 that beam having the polarization p_(∥). Again, in that embodimentand at the writing mode the polarization switch 70 changes thepolarization p_(∥) to the polarization p_(⊥). Thus, the radiation beamemerging from the polarization switch 70 has the polarization p_(⊥) andpropagates to the first polarizing beam splitter 9 via the collimatorlens 8: the splitter 9 reflects that beam toward the detector 24.

As shown in FIG. 10B (reading mode), the radiation beam 4 has thepolarization p_(∥) and therefore the polarizing beam splitter 9transmits that beam toward the collimator lens 8. The collimated beam 17with the polarization p_(∥) enters the power switch 203 and is enteringthe polarization switch 70. In that embodiment and at the reading modethe polarization switch 70 does not change the polarization. Thus, theradiation beam entering the second polarizing beam splitter 76 has thepolarization p_(∥): the splitter 76 transmits that beam toward theobjective lens 10 via the plate 75. After reflection on the informationlayer 2, the beam propagates to the beam splitter 76 via the objectivelens 10 and the plate 75, so that the beam has now the polarizationp_(⊥). Therefore, the beam splitter 76 reflects that beam toward themirror 72 via the plate 72. The mirror 72 then reflects the beam towardthe beam splitter 76, again via the plate 74, so that the beam has nowthe polarization p_(∥). Consequently, the beam splitter 76 transmitsthat beam toward the second mirror 71 via the plate 73. The mirror 71then reflects the beam toward the beam splitter 76, again via the plate73, so that the beam has now the polarization p_(⊥). Thus, the beamsplitter reflects toward the polarization switch 70 that beam having thepolarization p_(⊥). Again, in that embodiment and at the reading modethe polarization switch 70 does not change the polarization. Thus, theradiation beam emerging from the polarization switch 70 has thepolarization p_(⊥) and propagates to the first polarizing beam splitter9 via the collimator lens 8: the splitter 9 reflects that beam towardthe detector 24.

The distances h_(writing) and h_(reading) depends on the designparameters of the second beam splitter 76, the mirrors 71 and 72, andthe plates 73, 74 and 75. It has been found that the distancesh_(writing) and h_(reading) are given, in that embodiment, by thefollowing equation:

$\begin{matrix}{\frac{h_{writing}}{h_{reading}} = {- \frac{f_{1}}{f_{2}}}} & (8)\end{matrix}$where “f₁” and “f₂” are the image focal lengths of the mirrors 71 and72, respectively.

By way of illustration only, Table III shows values of the focal lengthsf₁ and f₂ at the writing and reading modes and the corresponding heightsh_(writing) and h_(reading) according to Equations (8a) and (8b).

TABLE III f₁ [in mm] f₂ [in mm] h_(writing) [in mm] h_(reading) [in mm]Writing mode +50 −43.75 1.6 N/A Reading mode N/A N/A N/A 1.4

Therefore, and with reference to Table I, by a proper choice of thefocal lengths f₁ and f₂ it is possible to transform the collimatedradiation beam 17 into the radiation beam 25 where the rim intensity ofthe beam 25 entering the objective lens 10 equals, at the writing mode,I_(rim,writing) so that the light power P equals P_(writing) and, at thereading mode, I_(rim,reading) so that the light power P equalsP_(reading). As already mentioned with reference to Table I, the ratioP_(writing)/P_(reading) approximately equals 125% in the ideal casewhere there is no light transmission loss in the optical scanning device1 is provided with the power switch 203 shown in FIG. 9.

It is noted that the scanning spot power switch 203 is arranged betweenthe collimator lens 8 and the objective lens 10, thereby forming atelescope-like arrangement having both object and image conjugates atthe infinity.

It is to be appreciated that numerous variations and modifications maybe employed in relation to the embodiments described above, withoutdeparting from the scope of the invention that is defined in theappended claims.

As an alternative to either the first or second embodiment of thescanning spot power switch, the entrance or exit face of theelectrowetting device may be designed for serving, e.g., as a lens suchas the collimator lens or as a diffractive structure.

1. An optical scanning device for scanning an information layer by meansof a radiation beam in a writing mode and a reading mode, the devicecomprising: a radiation source for emitting said radiation beam, anobjective lens having an optical axis, for converging said radiationbeam so as to form a scanning spot in the position of said informationlayer, and a scanning spot power switch arranged in the optical path ofsaid radiation beam, for switching the light power of said scanning spotbetween a first light power level at the writing mode and a second,lower light power level at the reading mode, characterized in that saidscanning spot power switch is further arranged for switching the size ofthe cross-section of said radiation beam between a first size at thewriting mode and a second, larger size at the reading mode so as toswitch the rim intensity of said radiation beam between a first rimintensity level at the writing mode and a second, higher rim intensitylevel at the reading mode, thereby switching the light power of saidscanning spot between said first light power level at the writing modeand said second light power level at the reading mode.
 2. An opticalscanning device according to claim 1, further including a collimatorlens arranged between said radiation source and said scanning spot powerswitch and wherein said scanning spot power switch forms atelescope-like arrangement having a switchable transverse magnificationbetween the writing and reading modes so that the cross-section of saidradiation beam has said first size at the writing mode and said secondsize at the reading mode.
 3. An optical scanning device according toclaim 1, wherein said scanning spot power switch includes avariable-focus liquid lens having a first meniscus and a second meniscusthe shapes of which are electrically adjustable such that thecross-section of said radiation beam has said first size at the writingmode and said second size at the reading mode.
 4. An optical scanningdevice according to claim 1, wherein said radiation beam has either afirst polarization or a second, different polarization and wherein saidscanning spot power switch includes: a polarizing beam splitter capableof transmitting and reflecting said radiation beam depending on itspolarization, a first mirror arranged on one side of said polarizingbeam splitter and a second mirror arranged on another side of saidpolarizing beam splitter, a first quarter-wavelength plate arrangedbetween said polarizing beam splitter and said first mirror, a secondquarter-wavelength plate arranged between said polarizing beam splitterand said second mirror, a third quarter-wavelength plate arrangedbetween said polarizing beam splitter and said objective lens, apolarization switch arranged in the optical path of said radiation beam,capable of changing the polarization of said radiation beam between saidfirst and second polarizations such that the cross-section of saidradiation beam has said first size at the writing mode and said secondsize at the reading mode.
 5. An optical scanning device according toclaim 1, further including a detection system arranged for providing afocus error signal and/or a radial-tracking error signal and in that itfurther includes a servo circuit and an actuator responsive to saidfocus error signal and/or said radial-tracking error signal forcontrolling the positions of said scanning spot with respect to theposition of said information layer and/or of a track of said informationlayer which is to be scanned.
 6. An optical scanning device as claimedin claim 5, further including an information processing unit for errorcorrection.
 7. A scanning spot power switch suitable for an opticalscanning device for scanning an optical record carrier by means of aradiation beam at a writing mode and a reading mode, the power switchbeing arranged for switching the light power of said scanning spotbetween a first light power level at the writing mode and a second,lower light power level at the reading mode characterized in that it isfurther arranged for switching the size of the cross-section of saidradiation beam between a first size at the writing mode and a second,larger size at the reading mode so as to switch the rim intensity ofsaid radiation beam between a first rim intensity level at the writingmode and a second, higher rim intensity level at the reading mode,thereby switching the light power of said scanning spot between saidfirst light power level at said first mode and said second light powerlevel at said second mode.